This could be a multitude of issues. Take it to a transmission shop if you don't know how to trouble shoot it. I rebuild tyranny's, so there are to many things that can affect the tranny. Slipping frictions and steels. Slipping torque converter lock up clutch, low fluid levels. way to many items that it can be. Your going to have to bite the bullet on this one probably unless your a competent rebuilder and diagnostics mechanic.
The radiator could be stopped up causing transmission to overheat as well as motor
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